North Korean leader Kim Jong Un's uncle, the Pyongyang regime's de facto No 2 official, has probably been ousted from his posts, South Korea's state spy agency says.

In a report released at a meeting with lawmakers on Tuesday (local time), the National Intelligence Service said that two close confidants of 67-year-old Jang Song Taek appear to have been publicly executed in late November for "anti-party activities".

The intelligence agency concluded that the executions of Ri Yong-Ha and Jang Soo-Kil couldn't have taken place unless Jang, holding the position of vice chairman of the National Defence Commission, had lost his job.

Jang is the husband of the late leader Kim Jong Il's only full sibling, Kim Kyung Hui, and held a position in the secretive family hierarchy tantamount to regent after Kim Jong Il died in December 2010, leaving the not-yet-30-year-old Kim Jong Un in charge.

He has been a power broker in North Korea for years, with brothers occupying key positions in the military.

"If Jang was really overthrown, North Korea's power structure will be shaken greatly," Jung Chung-rae, a member of the parliamentary intelligence committee in Seoul, told reporters.

Andrei Lankov, a North Korea scholar at Seoul's Kookmin University, said that Jang's purge would be a sign that Kim Jong Un is solidifying his hold on power.

"It is a generation shift. The young forces are rising and pushing out the old. Kim Jong Un can't run the country when he has subordinates who are twice his age," said Lankov.

An added problem for Jang is that his wife, the young leader's aunt and the source of Jang's power, is reported to be extremely ill with cancer and liver disease said to a result of acute alcoholism. The couple have not been seen publicly for several weeks.

For decades, Jang has been viewed as a relative liberal in the hard-line Communist state, favouring economic reforms and a gradual opening of the country.

"The moderates and military-centered hard-line faction always had conflicts. This may have led to Jang's alleged fall," said Moon Hong-sik, a research fellow at the Institute for National Security Strategy.

Jang was last seen in North Korean media on November 6.

This is not the first time that Jang disappeared from public. In early 2004 Jang was purged by then-leader Kim Jong Il. Two years later, in 2006, Jang returned to work, and subsequently was promoted to his current post in 2010 by the ailing Kim, in part as regent to the young son.